1. Field of the Invention
The present invention relates to an optical diffusing plate that gives an anisotropic scattering of a linearly polarized light and has an excellent diffusion property in a scattering direction, and that is suitable for improving visibility, brightness etc. of a liquid crystal display etc. Moreover, the present invention relates to an optical element using the optical diffusing plate concerned.
2. Description of the Prior Art
Conventionally, an optical diffusing plate designed so that anisotropic scattering might be given to a linearly polarized light by making a domain with a refractive index anisotropy contained in a dispersed state in a base material has been known. As the optical diffusing plate concerned; an optical diffusing plate that comprises a combination of a thermoplastic resin and a low molecular weight liquid crystal, that comprises a combination of a low molecular weight liquid crystal and a photo-curable low molecular weight liquid crystal and that comprises a combination of a polyvinyl alcohol and a low molecule liquid crystal, have been known (U.S. Pat. No. 2,123,902 specification, WO 87/01822 official gazette, JP-A 9-274108).
What is expected in the above-mentioned optical diffusing plate is that supplying a linearly polarized light in a state difficult to be absorbed by a polarizing plate reduces an absorption loss, and consequently the brightness of a liquid crystal display is improved. It is expected using this method that the following conventional problems may be solved; a problem based on a large wavelength dependability of a cholesteric liquid crystal in the conventional absorption loss reduction systems in which a cholesteric liquid crystal layer and a quarter wavelength plate are used, and especially, problems that transmitted light is chromatic from oblique direction and that these cannot be applied to a reflected type liquid crystal display etc. However, in the above-mentioned conventional optical diffusing plate, there were problems of a difficulty in manufacturing, and of a practical usability in difficult handling with a poor functional stability, when applied in a liquid crystal display, etc.
As an optical diffusing plate in which the above-mentioned problem was solved, an optical diffusing plate using a birefringent film that contains minute domains with different birefringent characteristics in a dispersed state is proposed (JP-A 2000-187105). Although the above-mentioned problem is solved and excellent polarization characteristics are developed using this optical diffusion plate, the further improvement is desired about polarization characteristics.
An object of the present invention is to provide an optical diffusing plate that is easily manufactured and has outstanding thermal and chemical stability and is excellent in practical use or is excelled in polarization characteristics, and in which while brightness is increased as a result of supplying linearly polarized light with reduced absorption loss by polarizing plate, chromatic problem is rarely induced, and that is applicable to reflected type liquid crystal display etc. And another object of the present invention is to provide an optical element and further a liquid crystal display using the above-mentioned optical diffusing plate.
As a result of repeated examinations carried out wholeheartedly by the present inventors to solve the above-mentioned problems, it was found out, as is shown below, that the above-mentioned object was attained and the present invention was completed using an optical diffusing plate in which a side chain type liquid crystal copolymer is used as a minute domain contained in dispersed state in a birefringent film.
Namely, the present invention relates to an optical diffusing plate comprising a birefringent film and a minute domain with a birefringent characteristic different from the birefringent film in a dispersed state in the birefringent film,
the minute domain comprising a side chain type liquid crystal polymer comprising a monomer unit (a) containing a liquid crystalline fragment side chain and a monomer unit (b) containing a non-liquid crystalline fragment side chain, and
a refractive index difference (xcex94n1) between the birefringent film and the minute domain in a direction orthogonal to a direction of axis that gives a maximum transmittance of linearly polarized light being 0.03 or more, and a refractive index difference (xcex94n2) in a direction of axis that gives a maximum transmittance being 80% or less of the xcex94n1.
Since the minute domain and the birefringent film that contains the dispersed minute domain is formed by a polymer materials and a forming material has an excellent handling property, the birefringent film is easily manufactured using an optical diffusing plate of the present invention. Since the forming material has a thermal and chemical stability, it develops stable optical function and excels in practicality. Moreover, a linearly polarized light transmits in the direction of the axis (xcex94n2 direction) that gives a maximum transmitance of the linearly polarized light, while maintaining an excellent polarization state, and the linearly polarized light is scattered in the direction (xcex94n1 direction) orthogonal to the above-mentioned xcex94n2 direction based on a refractive index difference xcex94n1 between a birefringent film and a minute domain, and as a result the polarization state is relaxed or canceled.
And moreover since a side chain type liquid crystal copolymer contains a monomer unit (b), it becomes possible to provide a birefringent characteristic smaller than the one obtained using only a side chain type liquid crystal polymer consisting of a monomer unit (a) independently. Consequently, since the birefringent characteristic of a side chain type liquid crystal copolymer may be controlled to a desired value of birefringence, it becomes possible to manufacture an optical diffusing plate with outstanding polarization characteristic.
In the above-mentioned optical diffusing plate, it is preferable that a minute domain is distributed in a dispersed and state caused by phase separation, and that a length in xcex94n1 direction of the minute domain (direction orthogonal to the above-mentioned axial direction) is from 0.05 to 500 xcexcm.
The above-mentioned optical diffusing plate may be used as one layer, two or more layers may be used in laminated state so that the xcex94n1 direction of an upper layer and a lower layer may have a mutually parallel relationship in order to increase polarization characteristics.
Moreover, the present invention relates to an optical element comprising a laminated layer of at least one kind selecting from a polarizing plate and a retardation plate, and the above mentioned optical diffusing plate.
In the above-mentioned optical element, it is desirable that transmission axis of the polarizing plate and the xcex94n2 direction of the optical diffusing plate (the above-mentioned axial direction) have a mutually parallel relationship.
When a polarizing plate is configured to an optical diffusing plate so that the transmission axis may have a relationship parallel to the above-mentioned xcex94n2 direction, a linearly polarized light that has transmittance in the xcex94n2 direction transmits polarizing plate efficiently and a linearly polarized light that has transmittance in the above-mentioned xcex94n1 direction is scattered, and the polarization direction is transformed. Consequently since the linearly polarized light becomes to contain a linearly polarized light component that has a transmittance in the xcex94n2 direction, the component light is transmitted through the polarizing plate.
Furthermore, the present invention relates to a liquid crystal display comprising the above-mentioned optical diffusing plate or the above-mentioned optical element on one side or both sides of liquid crystal cells.
When the above-mentioned polarizing plate is used, an amount of the transmitted linearly polarized light is increased and an absorption loss is reduced simultaneously, the brightness of a transmitted type liquid crystal display etc. can be raised. Moreover, a possible chromatic problem based on high wavelength dependability as in cholesteric liquid crystal becomes rarely to be induced. Furthermore, the above-mentioned polarizing plate is easily applicable also in a reflected type liquid crystal display etc., and a liquid crystal display with outstanding brightness and visibility may be stably obtained.